The invention is directed to improvements in a method for sealing off an overpressure valve for a packaging container.
It is known for overpressure valves on packaging containers to allow gas produced by the product in the container--such as carbon dioxide in the case of coffee--to escape from the packaging container at a certain overpressure yet to prevent air and thus oxygen from coming into contact with the product. Such valves are generally equipped with a liquid sealant, such as silicone oil. The sealant disposed between the valve elements does not impair the opening of the valve, but in the closed position of the valve it prevents oxygen molecules from passing through the valve into the packaging container through tiny channels formed between uneven portions of the contacting surfaces of the valve elements. By a method disclosed in European Patent Document B 12 874, the liquid sealant is applied with a nozzle to an inlet or outlet end of the valve channel formed by foil elements; from there, it penetrates the channel by capillary attraction. It may sometimes happen, for instance if the applicator is defective or if the sealant supply is exhausted, that valves may not be equipped with sealant, so that their tightness does not meet requirements. Packaging containers thus provided with unsealed valves do not assure the necessary protection for the product, so that the product prematurely loses its quality or even spoils as a result of oxidation. Until now, it was not possible to check whether a valve was activated with sealing fluid. Although it has already been proposed that the presence of sealant in a valve be ascertained optically with the aid of a photoelectric cell, the results were not satisfactory.